Highly resistant and ductile fibre concrete

ABSTRACT

A concrete in which metallic fibres are dispersed, obtained through mixing with water a composition comprising:  
     a) a cement;  
     b) ultrafine elements with a pouzzolanic reaction;  
     c) granular elements distributed into two granular classes C 1 &gt;1 mm and &lt;5 mm and C 2  ranges from 5 to 15 mm;  
     d) cement additions;  
     e) an amount of water E added in the mixture;  
     f) a dispersant, and preferably a superplasticizer;  
     g) metallic fibres, in an amount maximum equal to 120 kg per m 3  of concrete,  
      the contents of the various components (a), (b), (C 1 ), (C 2 ), (d) and the amount of water E, expressed in volume, meeting the following relationships:  
     ratio 1: 0.50≦(C 2 )/(C 1 )≦1.20  
     ratio 2: 0.25≦[(a)+(b)+(d)]/[(C 1 )+(C 2 )]≦0.60  
     ratio 3: 0.10≦(b)/(a)≦0.30  
     ratio 4: 0.50≦E/[(a)+(b)+(d)]≦0.75  
     ratio 5: (d)/(a)≦0.20.  
     The invention applies to the manufacture of voussoirs without frameworks, of tiles and of elements of the plate, shell type or the like.

[0001] The invention relates to fibre-containing concretes and moreparticularly high performance hydraulic binder based concretes of thePortland cement type or similar and comprising metallic fibres.

[0002] The term “concrete” as used herein generally encompassesindiscriminately concretes, mortars or grouts, as will be referred to inthe remainder of the present text.

[0003] There are currently high performance non fibre-containingconcretes having the inconvenient of being brittle with low flexurestrengths.

[0004] Now, for many uses in civil engineering, and more particularlyfor coating tunnels using prefab voussoirs, small thicknesses aredesired, which requires high and very high performance concretes.Moreover, given the severe conditions to which such voussoirs aresubjected, a high flexure strength is desired, as well as a highconcrete workability and ductility.

[0005] This can only be satisfactorily achieved, from the economicalpoint of view, with armoured concretes having conventional passiveframeworks, even if these are very high performance concretes(compression strength Rc higher than 120 MPa). Such conventionalvoussoirs are subjected to complex and multidirectional stresses. Theymore particularly should have a flexure strength measured on prismaticsamples higher than 15 MPa, while being ductile.

[0006] The term ductility of a non resilient material such as concretemeans the ability thereof to be distorted beyond its yield strengthwithout suddenly break and, preferably, while having an increase instress or at least a plateau.

[0007] Moreover, it is desirable that such concretes have a variableconsistency, ranging from firm to autoconforming depending on theapplications.

[0008] There are currently recent technical solutions involving veryhigh performance concretes comprising metallic or organic fibres, whichare ductile or the technical performance of which could make it possibleto make members having the required features for tunnel voussoirs.

[0009] More particularly, patent applications WO-99/28267 and EP-934915propose very high performance fibre-containing concretes being able tomeet the desired technical performance. However, patent applicationWO-99/28267 discloses a concrete with a granular skeleton comprisingfine and ultrafine particles making it highly performant but expensivefor the required uses.

[0010] In patent application EP-934915, the above-mentioned requiredtechnical performance is reached, on the one hand, through very hardgranulates, such as calcined bauxite which is an expensive granulate, onthe other hand, by using very high amounts of fine and ultrafineparticles, which also makes the material expensive.

[0011] Besides the fact that such formulations are very expensive, theyrequire, for their implementation, the use of particular equipment forintroducing the fibres and mixing the fibre-containing concrete. It istherefore difficult to implement them in the conventional ready-to-useconcrete units with concrete delivery via a remote cement mixer truck.

[0012] Using prior art formulae does not allow to obtain satisfactorysolutions from the economical point of view for solving the occurringproblem, i.e. for producing a ductile concrete with a compressionstrength Rc higher than 120 MPa and a flexure strength Rfl higher thanor equal to 15 MPa (strengths Rc and Rfl are measured on cylindrical orprismatic samples), and comprising at maximum 120 kg metallic fibres perm³ of concrete and this, using the conventional granulates, such as theyare available in the classical ready-to-use concrete units or in theprefabrication units.

[0013] In order to obtain a concrete meeting such criteria, theApplicant has been led, for obtaining the desired mechanical performancewith much cheaper material costs, to use a formulation concept differingfrom the prior art wherein use is made of a granular skeleton of theappolonian type in the presence of fibres, and of a binder pasteoptimized from the mechanical and rheological point of view; the pasteamount in the concrete is determined by the loosening degree of thegranular skeleton required for obtaining the desired workability.

[0014] An object of the invention is therefore to provide a very highperformance concrete having metallic fibres with mean mechanicalfeatures (measured on cylindrical or prismatic samples, namely:

[0015] Rc>120 MPa, and

[0016] Rfl≧15 MPa,

[0017] while being ductile and comprising at maximum 120 kg metallicfibres per m³ of concrete and this, using the conventional granulates,such as they are available in the ready-to-use concrete facilities or inthe prefab element facilities.

[0018] Depending on the application type, such concretes would have aconsistency ranging from firm to autoconforming, the determination ofsuch a consistency occurring according to the DIN 1048 standard.

[0019] The present invention has therefore the aim of producing with theusual components of high performance concretes, a concrete havingmetallic fibres, with a metallic fibre content equal to or higher than120 kg/m³, such concretes allowing, in particular, to manufacturevoussoirs for tunnels without frameworks and having a thickness that canbe lower than 10 cm.

[0020] The composition of the concrete according to the invention allowsfor fibre-containing concretes to be produced with improved technicalperformance and costs compared to the prior art. More particularly, thecomposition may be extended to the use of any type of fibres in theconcrete. The resulting cost/performance ratios are more favorable thanthose of the fibre-containing concrete compositions known until now.

[0021] The aims of the present invention are reached to produce aconcrete comprising a hardened cement matrix in which metallic fibresare dispersed, obtained through mixing components comprising:

[0022] a) a cement with particles having a grain size D50 ranging from10 to 20 μm;

[0023] b) ultrafine elements with a pouzzolanic reaction, the elementparticles of which have a grain size D50 of maximum 1 μm;

[0024] c) granular elements distributed into two granular classes C₁ andC₂ defined as follows:

[0025] C₁: particles with a size higher than 1 μm and lower than 5 mm,

[0026] C₂: particles with a size ranging from 5 to 15 mm, and preferablyfrom 6 to 12 mm;

[0027] d) cement additions with a grain size D50 lower than or equal to100 μm;

[0028] e) an amount of water E added in the blend;

[0029] f) a dispersant, preferably a superplasticizer, being present ina dry matter proportion ranging from 1.5 to 5% by volume based on thecement;

[0030] g) metallic fibres, in an amount maximum equal to 120 kg per m³of concrete and having an individual length l such that the concrete hasa l/φ ratio of at least 2, and preferably at least 3, 1 being the fibreindividual length and φ being the diameter of the biggest grain;characterized in that the contents of the various components (a), (b),(C₁), (C₂), (d) and the amount of water (E), expressed in volume, meetthe following relationships:

[0031] ratio 1: 0.50≦(C₂)/(C₁)≦1.20

[0032] ratio 2: 0.25≦[(a)+(b)+(d)]/[(C₁)+(C₂)]≦0.60

[0033] ratio 3: 0.10≦(b)/(a)≦0.30

[0034] ratio 4: 0.50≦E/[(a)+(b)+(d)]≦0.75

[0035] ratio 5: (d)/(a)≦0.20.

[0036] Advantageously, the following relationships are obtained for theratios 1, 3, 4 and 5 of the contents of the components (a), (b), (C₁),(C₂), (d) and the amount of water E, expressed in volume:

[0037] ratio 1: 0.60≦(C₂)/(C₁)≦1.0

[0038] ratio 3: 0.15≦(b)/(a)≦0.25

[0039] ratio 4: 0.55≦E/[(a)+(b)+(d)]≦0.70

[0040] ratio 5: (d)/(a)≦0.15

[0041] whatever the consistency of the resulting concrete is.

[0042] Depending on the desired concrete consistency, from firm toautoconforming, the ratio 2 of the components (a), (b), (d), (C₁) and(C₂) meets the following relationships:

[0043] 1) ratio 2: 0.25≦[(a)+(b)+(d)]/[(C₁)+(C₂)]≦0.45 in the case of afirm to fluid consistency,

[0044] 2) ratio 2: 0.45≦[(a)+(b)+(d)]/[(C₁)+(C₂)]≦0.65 in the case of anautoconforming consistency,

[0045] the ratios 1, 3, 4 and 5 remaining unchanged whatever theconsistency is firm to fluid or autoconforming.

[0046] The presence of metallic fibres in the concrete compositionaccording to the invention allows for voussoirs to be manufactured fortunnels with frameworks and also members such as tiles, plates, shellsor the like.

[0047] The cement (a) of the composition according to the invention isadvantageously a Portland cement. Preferably, the cement of thecomposition according to the invention is a Portland HTS cement, saidcement comprising at least 20% by weight of combined silica based on thecement weight. The cement may also be a calcium aluminate based cementor any hydraulic binder based on blast furnace slag or even anyhydraulic binder based on blends of such cements and/or slags.

[0048] The ultrafine elements with a pouzzolanic reaction (b) are knownin the art. They are generally selected amongst silica fumes, preferablysilica fumes from the zirconium industry or the silicon industry.

[0049] The granular elements (c) may be any granular elementsconventionally available for producing concretes. The granular elements(c) are sieved or ground gravels, sands or sand mixtures.

[0050] The cement additions (d) comprise fly ashes and/or chalkyfillers, and/or slags, and/or silica sands, more particularly quartzflour or ground fine chalks.

[0051] In a preferred embodiment, the cement particles (a) have a grainsize D50 of about 15 μm, the ultrafine elements with a pouzzolanicreaction (b) have a particle size D50 lower than 1 μm.

[0052] As to the metallic fibres, they can be more particularly selectedamongst steel fibres with a low carbon content (hypoeutectoid steels),steel fibres with a high carbon content (eutectoid and hypereutectoidsteels) having high mechanical strengths, alloyed or micro-alloyed steelfibres, amorphous steel fibres as well as stainless steel fibres.Preferably, steel fibres with low carbon content will be used, or steelfibres with high carbon content.

[0053] The amount of metallic fibres in the concrete is lower than orequal to 120 kg per m³ of concrete, generally from 20 to 120 kg/m³ ofconcrete, and preferably from 40 to 100 kg per m³ of concrete.

[0054] Expressed in volume, the metallic fibres generally account for1.5% or less of the concrete volume.

[0055] The individual length l of the metallic fibres is generally atleast twice and preferably at least three times the size of the biggestgrain.

[0056] The steel fibres may optionally be coated with a non ferrousmetal such as copper, zinc, nickel or the alloys thereof.

[0057] Varying geometry fibres may be used; they may be crenellated,ondulated or crocheted at the ends.

[0058] One may also adjust the fibre roughness and/or use fibres with avarying cross-section.

[0059] The composition according to the invention also includes adispersant, preferably a superplasticizer, present in a dry matterproportion from 1.5 to 5%, and preferably from 2.5 to 3.5% by volumebased on the cement.

[0060] The superplasticizers are conventional components of theconcretes with the object to improve the concrete rheology. Amongst suchsuperplasticizers, the polyoxyethylenated phosphonates POE, the polyoxpolycarboxylates PCP and the blends thereof are particularlyrecommended. Such superplasticizers are commercially available products;examples include the OPTIMA 100®, PREMIA 100® and OPTIMA 175® productssold by CHRYSO.

[0061] The concretes according to the invention may also comprisevarious other additives, including colouring pigments, dispersants,anti-foam agents, antisweating or antideposition agents, settingaccelerators or aqueous emulsions of organic products well known to theman of the art.

[0062] The concretes according to the invention may also comprise shortfibres (with a length lower than 2 mm, preferably maximum 1 mm) ofpolyvinyl alcohol, polyacrylonitrile, high density polyethylene, aramidpolyamid or polypropylene.

[0063] The concrete is prepared using any method known to the man of theart, including mixing the solids components and water, shaping(moulding) and then hardening.

[0064] Generally, the resulting concrete maturation may be performed:

[0065] either in the form of a storage at 20° C. and more than 90%relative humidity,

[0066] or through a thermal treatment directly after positioning in themould,

[0067] or through a thermal treatment from a predetermined time beforewhich it will have been stored at 20° C. and more than 90% relativehumidity directly after positioning in the mould.

[0068] Such a thermal treatment will occur at a temperature ranging from20° C. to 100° C.

[0069] The resulting concretes according to the present invention show:

[0070] a flexure strength Rfl measured on prismatic samples, higher thanor equal to 15 MPa,

[0071] a compression strength Rc measured on cylindrical samples, higherthan or equal to 120 MPa,

[0072] said flexure strength Rfl and compression strength Rc beingevaluated at the 28 day time.

[0073] Concrete Corresponding to Examples 1 and 2

[0074]1) Raw Materials

[0075] In order to give full meaning to the comparisons being carriedout, the same components as indicated hereafter have been implementedfor all the examples.

[0076] cement:

[0077] high silica content HTS type Portland cement (CPA CEM I 52.5)from LAFARGE CIMENTS FRANCE,

[0078] 50 type Portland cement (according to the CSA Canadian standard)from LAFARGE CORPORATION.

[0079] granular elements:

[0080] GRENADE 0/4 alluvial sand from LAFARGE GRANULATS FRANCE,

[0081] LE TERTRE 6/10 gravels (silicated sedimentary rock) from LAFARGEGRANULATS FRANCE,

[0082] GSI 0/0.315 siliceous sand from SIFRACO,

[0083] 6/12 granite gravel,

[0084] 0/5 siliceous sand,

[0085] 0/0.5 siliceous sand

[0086] cement additions:

[0087] fly ashes from SUNDANCE

[0088] ultrafine elements with a pouzzolanic reaction:

[0089] silica fumes: ELKEM 940 U glass microsilica

[0090] SKW silica fumes

[0091] superplasticizer builders:

[0092] polyoxyethylenated phosphonate (POE), OPTIMA 100®, from CHRYSO,FRANCE

[0093] mixture of polyoxyethylenated phosphonate (POE) and polyoxpolycarboxylate (PCP), OPTIMA 175®, from CHRYSO, FRANCE

[0094] metallic fibres:

[0095] Fibres A: DRAMIX RC 80-60 LC steel fibres supplied by BEKAERT inthe form of platelets consisting in about thirty pasted fibres(crocheted cylindrical fibres, low carbon content, length I_(f)=60 mmand diameter d_(f)=0.8 mm).

[0096] Fibres B: DRAMIX RC 80-60 HC steel fibres supplied by BEKAERT,similar to fibres A but with high carbon content.

[0097] Fibres C: Steel fibres supplied by NOVOCON (ondulated fibres witha rectangular section I_(f)=50 mm, a_(f)=2.5 mm, b_(f)=0.4 mm).

[0098] 2) Method of Preparation

[0099] In example 1, the production of concrete is carried out in alaboratory with a SKAKO mixer. During the preparation step of theconcrete, the components are mixed in the following order:

[0100] Introducing the granulates into the mixer,

[0101] Introducing the wetting water for 30 seconds,

[0102] mixing the wet granulates for 30 seconds,

[0103] standing for 4 minutes,

[0104] introducing the binders for 30 seconds,

[0105] blending for one minute,

[0106] introducing the mixing water and the builders,

[0107] mixing the concrete for 3 minutes,

[0108] introducing half of the metallic fibres while distributing themon the concrete surface,

[0109] starting the mixer and introducing the other half of the fibresfor 30 seconds, and

[0110] mixing the concrete for 1 minute.

[0111] In example 2, an industrial mixer is used. The method ofpreparation of the concrete is as follows:

[0112] introducing all the components except the fibres,

[0113] dry mixing for 1 minute,

[0114] introducing the water and the superplasticizer,

[0115] mixing for 5 minutes,

[0116] introducing the fibres, and

[0117] mixing for 1 minute.

[0118] The moulds are then filled, and then vibrated for the concreteswith a firm to fluid consistency, and with no vibration for theconcretes with an autoconforming consistency.

[0119] 3) Maturation

[0120] The samples are either immediately subjected to a thermaltreatment as previously defined, or stored under water at 20° C., andthen optionally subjected to a thermal treatment as previously defined,from a predetermined time.

[0121] Measuring Methods

[0122] 1) Spreading Measure

[0123] For concretes with low fluidity, the measure principle consistsin measuring the diameter of the concrete disk formed after the releasedconcrete has been subjected to shocks. The spreading measuring method isdescribed in the DIN 1048 standard.

[0124] For concretes with high fluidity, the same method is used, butwith no shocks.

[0125] For all the experiments, the spreading measurements are conductedafter the addition of the metallic fibres. The different consistenciescorrespond to the following spreading degrees:

[0126] firm consistency: DIN spreading with shocks lower than or equalto 350 mm,

[0127] plastic consistency: DIN spreading with shocks from 350 to 450mm,

[0128] fluid consistency: DIN spreading with no shocks from 450 to 600mm,

[0129] autoconforming consistency: DIN spreading with no shocks higherthan or equal to 600 mm.

[0130] The spreading measurements are performed either already at theend of the concrete production at (to), or one hour after the end of theconcrete production at (to +1 hour), which corresponds to a common usepratical duration.

[0131] 2) Measurement of the Flexure and Compression MechanicalStrengths

[0132] The measuring principle consists in determining the values of

[0133] flexure strength Rfl on prismatic samples of 10 mm×10 mm×40 mmdimensions with 4-point flexure trials, according to the operation modedefined by NF P 18-409 standard,

[0134] compression strength Rc on cylindrical samples of followingdimensions: φ=11 cm×h=22 cm, according to the operation mode defined byNF P 18-406 standard.

EXAMPLE 1

[0135] Various fibre-containing concretes according to the inventionhave been prepared, designated hereafter by R2 and O3 to O5.

[0136] Table 1 represents the compositions (expressed in kg/m³) of theconcretes R2, O3 to O5, as well as the ratio values 1 to 5 of thecontents of the components (a), (b), (C1), (C2), (d) and E such aspreviously defined. TABLE 1 Components kg/m³ Type Nature R2 O3 O4 O5Gravel 6/10 Le Tertre 892 744 665 575 Sand 0/4 Grenade 498 428 386 509Fine sand 0/0.315 GSI 326 362 328 290 Cement CEM I 52.5 482 622 723 725HTS Le Teil Fumed silica 940 U ELKEM 67 87 101 101 Water Efficient water120 137 159 159 Superplasticizer OPTIMA 100 14.9 18.7 21.7 21.7 Metallicfibres RC 80-60 LC — 80 — 80 DRAMIX Metallic fibres RC 80-60 HC 80 — 80— DRAMIX Ratio 1 1.11 0.97 0.95 0.74 Ratio 2 0.28 0.40 0.51 0.52 Ratio 30.20 0.20 0.20 0.20 Ratio 4 0.67 0.59 0.59 0.59 Ratio 5 0 0 0 0

[0137] The ratio values 1 to 5 meet the required specifications.

[0138] Since the compositions of the concretes R2, O3 to O5 do notcomprise any cement additions (d), the ratio value 5 is nihil.

[0139] Such concretes comprise less than 120 kg of metallic fibres perm³.

[0140] The Theological and mechanical performance of such concretes isshown in table 2, as well as in FIGS. 1 and 2. TABLE 2 Experimentresults Expressed Typical nature in R2 O3 O4 O5 DIN spreading with noshocks Mm — 480 640 690 (to) DIN spreading with shocks (to) Mm 330 550 —— DIN spreading with no shocks Mm — — — 600 (to + 1 hour) Compressionstrength at 28 days MPa 141 148 146 144 Flexure strength at 28 days Mpa19.3 16.3 21.1 16.3

[0141] According to this table 2, all the concretes (R2, O3 to O5) meetthe following specifications:

[0142] a flexure strength Rfl at 28 days higher than 15 MPa, and

[0143] a compression strength Rc at 28 days higher than 120 MPa.

[0144] Table 2 also shows that such specifications are satisfactory forfirm, fluid and autoconforming concretes, respectively R2, O3, O4 andO5. The autoconforming concretes are concretes having a spreading withno shocks higher than 600 mm. The spreading results of the concretedesignated by reference O5 show that one can produce an autoconformingconcrete whilst the rheology is maintained for at least one hour.

[0145]FIGS. 1 and 2 each represent three curves obtained by 4-pointflexure trials, with in ordinates the stress values (kN) and inabscissas the shifting values respectively corrected for the concretesamples O3 and O5.

[0146] The trials are conducted according to the NF P 18-409 standard;the shifting values are subjected to a correction according to the rulesof the art well known to the man of the art, due to the shiftingoccurring during the trial implementation.

[0147] The three curves each correspond to a trial performed on a sample(three samples tested).

[0148]FIGS. 1 and 2 show that the concretes O3 and O5 have a ductilebehavior in flexure.

EXAMPLE 2

[0149] As in example 1, a fibre-containing concrete according to theinvention is prepared, designated hereafter as O6, in which cementadditions (fly ashes) have been added. The composition of such aconcrete O6 is shown in table 3 hereunder. TABLE 3 Components kg/m³ TypeNature O6 Gravel 6/12 mm granit 830 sand 0/5 mm siliceous 692 Fine sand0/05 mm siliceous 266 Cement Type 50 EXSHAW Portland 468 Flying ashesSundance 43 Fumed silica SKW 56 Water Efficient water 109Superplasticizer OPTIMA 175 21.5 Metallic fibres NOVOCON 40 Ratio 1 0.86Ratio 2 0.29 Ratio 3 0.18 Ratio 4 0.56 Ratio 5 0.14

[0150] This table shows that the ratio values 1 to 5 meet the requiredspecifications.

[0151] The results of the compression trials show that such a concretehas, at 28 days, a compression strength of 132 MPa.

1. A concrete in which metallic fibres are dispersed, obtained throughmixing with water a composition comprising: a) a cement with particleshaving a grain size D50 ranging from 10 to 20 μm; b) ultrafine elementswith a pouzzolanic reaction, the element particles of which have a grainsize D50 of maximum 1 μm; c) granular elements distributed into twogranular classes C₁ and C₂ defined as follows: C₁: particles with a sizehigher than 1 μm and lower than 5 mm, C₂: particles with a size rangingfrom 5 to 15 mm, and preferably from 6 to 12 mm; d) cement additionswith a grain size D50 lower than or equal to 100 μm; e) an amount ofwater E added in the blend; f) a dispersant, preferably asuperplasticizer, being present in a dry matter proportion ranging from1.5 to 5% by volume based on the cement; g) metallic fibres, in anamount maximum equal to 120 kg per m³ of concrete and having anindividual length l such that the concrete has a l/φ ratio of at least2, and preferably at least 3, l being the fibre individual length and φbeing the diameter of the biggest grain; characterized in that thecontents of the various components (a), (b), (C₁), (C₂), (d) and theamount of water (E), expressed in volume, meet the followingrelationships: ratio 1: 0.50≦(C₂)/(C₁)≦1.20 ratio 2:0.25≦[(a)+(b)+(d)]/[(C₁)+(C₂)]≦0.60 ratio 3: 0.10≦(b)/(a)≦0.30 ratio 4:0.50≦E/[(a)+(b)+(d)]≦0.75 ratio 5: (d)/(a)≦0.20.
 2. A concrete accordingto claim 1, characterized in that: a) the cement particles (a) have agrain size D50 of about 15 μm, b) the ultrafine elements with apouzzolanic reaction (b) have a particle size D50 lower than 1 μm.
 3. Aconcrete according to claim 1 or 2, characterized in that the followingrelationship is obtained for ratio 2 of the contents of the components(a), (b), (C₁), (C₂), (d), expressed in volume: ratio 2:0.25≦[(a)+(b)+(d)]/[(C₁)+(C₂)]≦0.45 in the case when the resultingconcrete has a firm to fluid consistency.
 4. A concrete according toclaim 1 or 2, characterized in that the following relationship isobtained for ratio 2 of the contents of the components (a), (b), (C₁),(C₂), (d), expressed in volume: ratio 2:0.45≦[(a)+(b)+(d)]/[(C₁)+(C₂)]≦0.60 in the case when the resultingconcrete has an autoconforming consistency.
 5. A concrete according toany one of preceding claims, characterized in that the followingrelationships are obtained for ratios 1, 3, 4, 5, of the contents of thecomponents (a), (b), (C₁), (C₂), (d), and the amount of water E,expressed in volume: ratio 1: 0.60<(C2)/(C₁)≦1.0 ratio 3:0.15≦(b)/(a)≦0.25 ratio 4: 0.55≦E/[(a)+(b)+(d)]≦0.70 ratio 5:(d)/(a)≦0.15 whatever the consistency of the resulting concrete is.
 6. Aconcrete according to any one of preceding claims, characterized in thatthe cement is a high silica content cement, said cement comprising atleast 20% by weight of combined silica based on the cement weight.
 7. Aconcrete according to any one of preceding claims, characterized in thatthe amount of metallic fibres in the concrete ranges from 20 to 120kg/m³, and preferably from 40 to 100 kg per m³ of concrete.
 8. Aconcrete according to any one of preceding claims, characterized in thatthe metallic fibres are steel fibres.
 9. A concrete according to claim8, characterized in that the steel fibres have a high carbon contentranging from 0.7% to 0.8%.
 10. A concrete according to claim 8,characterized in that the steel fibres have a low carbon content, lowerthan or equal to 0.1%.
 11. A concrete according to any one of precedingclaims, characterized in that it also comprises short fibres ofpolyvinyl alcohol, polyacrylonitrile, high density polyethylene, aramidpolyamide or polypropylene.
 12. A concrete according to any one ofpreceding claims, characterized in that it comprises a superplasticizerpresent in a dry matter proportion ranging from 1.5 to 5% by volume, andpreferably 2.5 to 3.5% by volume based on the cement.
 13. A concreteaccording to any one of preceding claims, characterized in that thesuperplasticizer is selected amongst the polyoxyethylenatedphosphonates, the polyox polycarboxylates and the mixtures thereof. 14.A concrete according to any one of preceding claims, characterized inthat it shows: a flexure strength Rfl measured on prismatic samples,higher than or equal to 15 MPa, a compression strength Rc measured oncylindrical samples, higher than or equal to 120 MPa, said flexurestrength and compression strength being evaluated at the end of a 28 daytime.
 15. A concrete according to any one of preceding claims,characterized in that the ultrafine elements with a pouzzolanic reaction(b) comprise silica fumes.
 16. A concrete according to any one ofpreceding claims, characterized in that the granular elements (c) aresieved or ground granulates or mixtures of granulates.
 17. A concreteaccording to any one of preceding claims, characterized in that thecement additions (d) are fillers, more particularly quartz flour or hardchalk or fly ashes or slags.
 18. Voussoirs, tiles, plate or shellelements, characterized in that they are made from a concrete accordingto any one of claims 1 to
 17. 19. A prefab element characterized in thatit is made from a concrete such as defined according to any one ofclaims 1 to
 18. 20. A concrete according to any one of claims 1 to 19,characterized in that it is produced in a ready-to-use concrete plant.21. A concrete according to any one of claims 1 to 19, characterized inthat it is produced in a prefabrication plant.
 22. A method forproducing a concrete in which metallic fibres are dispersed, comprisingmixing an amount of water E with a composition comprising: a) a cementwith particles having a grain size D50 ranging from 10 to 20 μm; b)ultrafine elements with a pouzzolanic reaction, the element particles ofwhich have a grain size D50 of maximum 1 μm; c) granular elementsdistributed into two granular classes C₁ and C₂ defined as follows: C₁:particles with a size higher than 1 μm and lower than 5 mm, C₂:particles with a size ranging from 5 to 15 mm, and preferably from 6 to12 mm; d) cement additions with a grain size D50 lower than or equal to100 μm; e) a dispersant, preferably a superplasticizer, present in a drymatter proportion ranging from 1.5 to 5% by volume based on the cement;f) metallic fibres in an amount maximum equal to 120 kg per m³ ofconcrete and having an individual length l such that the concrete has al/φ ratio of at least 2, and preferably of at least 3, l being theindividual length of the fibres and φ being the diameter of the biggestgrain;  characterized in that the contents of the various components(a), (b), (C₁), (C₂), (d) and the amount of water (E), expressed involume, meet the following relationships: ratio 1: 0.50≦(C₂)/(C₁)≦1.20ratio 2: 0.25≦[(a)+(b)+(d)]/[(C₁)+(C₂)]≦0.60 ratio 3: 0.10≦(b)/(a)≦0.30ratio 4: 0.50≦E/[(a)+(b)+(d)]≦0.75 ratio 5: (d)/(a)≦0.20.
 23. A methodaccording to claim 1, characterized in that: a) the cement particles (a)have a grain size D50 of about 15 μm, and b) the ultrafine elements witha pouzzolanic reaction (b) have a particle size D50 lower than 1 μm. 24.A method according to claim 22 or 23, characterized in that thefollowing relationship is obtained for ratio 2 of the contents of thecomponents (a), (b), (C₁), (C₂), (d), expressed in volume ratio 2:0.25≦[(a)+(b)+(d)]/[(C₁)+(C₂)]≦0.45 in the case when the resultingconcrete has a firm to fluid consistency.
 25. A method according toclaim 22 or 23, characterized in that the following relationship isobtained for ratio 2 of the contents of the components (a), (b), (C₁),(C₂), (d), expressed in volume: ratio 2:0.45≦[(a)+(b)+(d)]/[(C₁)+(C₂)]≦0.60 in the case when the resultingconcrete has a autoconforming consistency.
 26. A method according to anyone of claims 22 to 25, characterized in that the followingrelationships are obtained for ratios 1, 3, 4, 5, of the contents of thecomponents (a), (b), (C₁), (C₂), (d), and the amount of water E,expressed in volume: ratio 1: 0.60≦(C₂)/(C₁)≦1.0 ratio 3:0.15≦(b)/(a)≦0.25 ratio 4: 0.55≦E/[(a)+(b)+(d)]≦0.70 ratio 5:(d)/(a)≦0.15 whatever the consistency of the resulting concrete is. 27.A method according to any one of claims 22 to 26, characterized in thatthe cement is a high silica content cement, said cement comprising atleast 20% by weight of combined silica based on the cement weight.
 28. Amethod according to any one of claims 22 to 27, characterized in thatthe amount of metallic fibres in the concrete ranges from 20 to 120kg/m³, and preferably from 40 to 100 kg per m³ of concrete.
 29. A methodaccording to any one of claims 22 to 28, characterized in that themetallic fibres are steel fibres.
 30. A method according to claim 29,characterized in that the steel fibres have a high carbon contentranging from 0.7% to 0.8%.
 31. A method according to claim 29,characterized in that the steel fibres have a low carbon content, lowerthan or equal to 0.1%.
 32. A method according to any one of claims 22 to31, characterized in that it also comprises short fibres of polyvinylalcohol, polyacrylonitrile, high density polyethylene, aramid polyamideor polypropylene.
 33. A method according to any one of claims 22 to 32,characterized in that it comprises a superplasticizer, present in a drymatter proportion ranging from 1.5 to 5% by volume, and preferably 2.5to 3.5% by volume based on the cement.
 34. A method according to any oneof claims 22 to 33, characterized in that the superplasticizer isselected amongst the polyoxyethylenated phosphonates, the polyoxpolycarboxylates and the mixtures thereof.
 35. A method according to anyone of claims 22 to 34, characterized in that the concrete shows: aflexure strength Rfl measured on prismatic samples, higher than or equalto 15 MPa, and a compression strength Rc measured on cylindricalsamples, higher than or equal to 120 MPa, said flexure strength andcompression strength being evaluated at the end of a 28 day time.
 36. Amethod according to any one of claims 22 to 35, characterized in thatthe ultrafine elements with a pouzzolanic reaction (b) comprise silicafumes.
 37. A method according to any one of claims 22 to 36,characterized in that the granular elements (c) are sieved or groundgranulates or mixtures of granulates.
 38. A method according to oneclaims 22 to 37, characterized in that the cement additions (d) arefillers, more particularly quartz flour or hard chalk or fly ashes orslags.